Method of the purification of catholytes

ABSTRACT

A method of the purification of a catholyte in the electrolytic hydrodimerization of acrylonitrile for the production of adiponitrile which comprises subjecting acrylonitrile and lowboiling byproducts such as propionitrile contained in the catholyte to distillation to remove the same, then adjusting the pH of the aqueous phase in the remaining emulsified liquor to 3.0 or below to distribute the major portion of carboxylates contained in the aqueous phase into the oil phase mainly consisting of adiponitrile, and eliminating the carboxylates out of the system through the medium of the oil phase to effect reduction of the carboxylate concentration in the catholyte.

ite States atent 51 3 67 653 9 9 Sek o et a1. July 4, 1972 [54] METHOD OF THE PURIFICATION OF References Cited CATHOLYTES UNITED STATES PATENTS [72] Inventors: Maomi Seko, Tokyo; Shinsaku OgaWa, 3,597,331 8/1971 Ogawa et a1. ..203/43 Nobeoka; Ryozo Komori, Nobeoka; 3,280,168 10/1966 Campbell et al.... ...204/73 A Masaro Asuma, Nobeoka; Toshiro Isoya, 3,496,212 2/1970 Davidson et a1. ..203/39 Nobeoka; Takamasa Sakai, Nobeoka, all 2,548,369 4/1951 Harwood et a1. ....203/35 of Japan 3,429,783 2/1969 Campbell et al .203/36 [73] Assignee: gsaliir :(asei Kogyo Kabushiki Kaisha, Primary Examiner Norman Yudkoff Assistant ExaminerDavid Edwards 22 W 12, 1970 Attorney-Flynn & Frishauf l 30 853 [57] ABSTRACT 21 A .N 1 pp 0 A method of the purification of a catholyte in the electrolytic hydrodimerization of acrylonitrile for the production of [30] Foreign Application Priority Data adiponitrile which comprises subjecting acrylonitrile and lowboiling byproducts such as propionitrile contained in the May 1, 1969 Japan ..44/33244 catholyte to distillation to remove the Same, the adjusting the pH of the aqueous phase in the remaining emulsified liquor to [52] US. Cl ..203/34, 203/352,O2i)3]/343\, 30 or below to distribute the major portion of carboxylates 51 I t Cl B01; 3 34 contained in the aqueous phase into the oil phase mainly con- E i 9 43 Q sisting of adiponitrile, and eliminating the carboxylates out of USED CATHOLYTE (ACRYLONlTRlLE,

PROPIONITRILE) BOTTOM LIQUOR (ADIPONITRILE) the system through the medium of the oil phase to effect reduction of the carboxylate concentration in the catholyte.

6 Claims, 1 Drawing Figure OIL PHASE (ADIPONITRILE) SEPARATOR P RAL E AQUEOUS PHASE (CATHOLYTE FOR RECYCLE) PATENTEBJUL 4 1972 3. 674,653

LOW- BOILING LIQUOR USED CATHOLYTE (ACRYLONITRILE,

PROPIONITRILE) l DISTILLATION TOWER OIL PHASE (ADIPONITRILE) HEATER FLOW METER BOTTOM LIQUOR (ADIPONITRILE) l7 SEPARATOR |6 PUMP FILTER 3 7 PH ADJUSTMENT TANK AQUEOUS PHASE (CATHOLYTE FOR RECYCLE) METHOD OF THE PURIFICATION OF CATHOLYTFS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of eliminating carboxylates in the catholyte in the electrolytic hydrodimerization of acrylonitrile for the production of adiponitrile out of the system to purify the catholyte. More particularly, it is concerned with a method of removing carboxylates in the catholyte by adjusting pH of the emulsified catholyte from which acrylonitrile and the by-produced propionitrile have been in advance removed to 3.0 or below, to distribute the carboxylates contained in the aqueous phase into the oil phase mainly consisting of adiponitrile, and then eliminating the carboxylates out of the system through the medium of the oil phase.

2. Description of the Prior Art- It has been heretofore known as disclosed, for example, in Belgian Pat. Nos. 699,928 and 623,691 that adiponitrile is produced by conducting electrolytic hydrodimerization using as the catholyte an aqueous solution containing acrylonitrile and a supporting electrolyte. According to the prior art methods, there is provided an electrolytic cell in which a pair of cathode and anode is divided by a diaphragm such as a membrane of a cation exchange resin, the sections defined by the cation exchange resin membrane and the anode and by the cation exchange resin membrane and the cathode being made the anode chamber and the cathode chamber, respectively. Acrylonitrile can be electrolytically hydrodimerized to form adiponitrile by applying a direct current between the two electrodes while supplying the anode chamber with an aqueous solution of mineral acid such as diluted sulfuric acid and the cathode chamber with the aforementioned catholyte. It is essential to prevent polymerization of the acrylonitrile contained in the catholyte during the electrolysis for assuring the yield of adiponitrile and conducting continued stable operation by prevention of the formation of deposits within the apparatus. It has been widely adopted to prevent the polymerization of acrylonitrile by the addition of a polymerization inhibitor to the catholyte. However, in these methods, it is difficult to effect complete inhibition of the polymerization of acrylonitrile in the catholyte. Especially mentioned are the polymers deposited on the surface of the cathode, though being in a minimal amount, which disturb flow of the catholyte and supply of the hydrogen ion and the acrylonitrile onto the surface of the cathode to be a cause of gradual reduction in the yield of adiponitrile.

In order to make stableoperation feasible for a long period of time, it is therefore necessary to prevent adhesion of minimal amounts of the polymers to the surface of the cathode as perfectly as possible.

SUMMARY OF THE INVENTION As a result of extensive investigations, we have confirmed that carboxylates contained in minimal amounts in the catholyte such as acrylic, acetic and propionic acids cause polymerization of the acrylonitrile. Accordingly, the polymerization as described above could be prevented by making the concentration of these carboxylates in the catholyte as low as possible.

It is an object of this invention to provide a novel method of the purification of a catholyte produced in the electrolytic hydrodimerization of acrylonitrile. Another object is to provide a method of producing adiponitrile from acrylonitrile by electrolytic hydrodimerization in a high yield. Other objects will be apparent by the descriptions hereinbelow.

As well known, the cyano group of nitriles is readily hydrolyzed in the presence of a strong acid or alkali to form carboxylates. There exists conditions sufficient to cause hydrolysis of the cyano group also in the above-mentioned catholyte. As a matter of fact, a strong acid in the order of four to five normal is supplied by the immigration of hydrogen ions from the anode chamber through the cation exchange resin membrane into the cathode chamber.

Consequently, acrylonitrile, adiponitrile and the byproducts such as propionitrile in the catholyte will be contacted upon the surface of the cation exchange resin membrane with the strong acid immigrated, thereby forming acrylic, adipic and propionic acids and the like respectively by the hydrolysis of their cyano groups. A variety of by-product nitriles such as succinonitrile, a-methylglutaronitrile, iminocyanocyclopentane and polyacrylonitrile which coexist in minimal amounts in the catholyte will also undergo hydrolysis with the strong acid to form corresponding carboxylic acids.

On the surface of the cathode there is concentrated tetralkylammonium ions from the supporting electrolyte employed to form a strong alkaline condition, which will also give rise to hydrolysis of the cyano group to form carboxylates.

These carboxylates when contained in minimal amounts in the catholyte will cause an increase in the formation of polymers depending upon the amounts of the former. For example, it was found that 100 mg. of the polymers per ton of adiponitrile were formed on average on the surface of the electrode when the catholyte contained propionic acid at. a concentration of ppm or less, whereas 3 g. of the polymers per ton of adiponitrile were deposited on average when the concentration of propionic acid was 300 ppm or less. The presence of carboxylates is also associated with increased formation of other by-products, for example the yield of byproduct propionitrile being on average 4 percent on the consumed acrylonitrile with propionic acid by-product at 50 ppm or below while being increased to 7 percent with propionic acid existing at 300 ppm. Other carboxylates also induce the polymerization of acrylonitrile as well as an increase in the formation of by-product propionitrile.

As described in detail above, carboxylates contained in minimal amounts in the catholyte hinder stable operation of the step of electrolytically hydrodimerizing acrylonitrile to produce adiponitrile. It is therefore necessary to achieve continuous removal of said acids from the catholyte to maintain their low concentrations in the catholyte.

In view of the above-described facts, extensive investigations were made by the present inventors on the removal of carboxylates to discover that the desired object could be achieved by the use of the partition coefficient of said carboxylates into adiponitrile. Thus, this invention relates to a method of the purification of a catholyte in the electrolytic hydrodimerization of acrylonitrile for the production of adiponitrile which comprises subjecting acrylonitrile and lowboiling by-products such as propionitrile existing in the catholyte to distillation to remove the same, then adjusting pH the of the aqueous phase in the remaining emulsified liquor to 3.0 or below, to distribute the major portion of carboxylates to the oil phase mainly consisting of adiponitrile and eliminating the carboxylates out of the system through the medium of the oil phase to effect reduction of the carboxylate concentration in the catholyte.

PREFERRED EMBODIMENTS OF THE INVENTION By way of illustration one suitable process scheme which may be employed is diagrammatically represented in the attached flowsheet. Referring to the flowsheet, first, catholyte 11 is passed to a distillation tower 1, through which the lowboiling components such as acrylonitrile and propionitrile are removed. A portion of the low-boiling distillate is refluxed to the top of the tower 13 and the remaining portion 14 is passed to the recovery step. Bottom liquor 15 from the distillation tower l, which consists of an oil phase mainly comprising adiponitrile and an aqueous phase mainly comprising a supporting electrolyte, is passed by means of a pump 9 to a filler 2 in which it is filtered l6 and then the aqueous phase is adjusted to a pH of 3.0 or below, preferably 2.0-3.0 in a pH adjustment tank 3. A heater is represented by 10. The pH adjustment is made by supplying sulfuric acid 20 at a predetermined rate from a sulfuric acid tank 4 by means of a metering pump 5 through a flow meter 6. The bottom liquor 17 from the pH adjustment tank is passed by means of a pump 7 to a separation tower 8, in which it is settled and separated into two phases. The oil layer thus separated mainly consists of adiponitrile and the aqueous phase mainly comprised of the supporting electrolyte and water. The solutions are respectively withdrawn by overflow out of the separation tower 8 (18 and 19). The oil phase 18 is withdrawn from the system and fed to the subsequent purification step. The aqueous phase 19 is recycled to the catholyte tank again for use in the electrolysis.

The carboxylates contained in the catholyte, the major portion of which exists under neutral conditions in the aqueous phase owing to nearly complete dissociation represented by the right side of the Equation l RCOOH RCOO'+H (I) wherein R represents alkyl. As the pH is reduced, the equilibrium is favored to the left side of the Equation (1), thereby increasing a free carboxylic acid in the solution and the major portion thereof being partitioned into the oil layer. Accordingly, free carboxylic acids contained in the bottom liquor, when adjusted to a pH of 3.0 or below, preferably 2.0-3.0, is distributed from its aqueous phase into its oil phase for the most part and withdrawn out of the system 18.

The method of this invention may be applied not only to the bottom liquor 15 from the distillation tower 1 but also any liquor consisting of an oil phase and an aqueous phase. However, if it contains low-boiling components such as acrylonitrile and propionitrile, they will be further hydrolyzed respectively to fonn acrylic and propionic acids and the like. It is therefore desirable to remove these low-boiling components in advance. pH in the pH adjustment tank is controlled within such a range that it is sufficiently low to lower the degrees of dissociation of the carboxylic acids as long as it is not so low as to cause hydrolysis of adiponitrile contained in the catholyte. In fact, reduction of the pH to 3.0 or lower can lead to increase in the partition coefiicients of carboxylates, mainly propionic acid into the oil phase. Moreover, maintaining the pH at 2.0 or higher can prevent hydrolysis of the adiponitrile to a practically negligible extent.

The pH adjustment is easily carried out by continuously dropping an acid with stirring into the adjustment tank. Any strong acid may be employed for this purpose such as hydrochloric, nitric, sulfuric, an alkylsulfuric or phosphoric acid. Usually, it is preferable to use sulfuric and alkylsulfonic acids that are used in the electrolysis reaction. Most simply, pH can be easily adjusted by quantitatively dropping 20 percent sulfuric acid.

Although the object of this invention can be satisfactorily achieved by the procedures as set forth above, the amount of the oil layer as the medium for removing the carboxylates is chiefly limited depending upon the amount of adiponitrile produced in the electrolytic cell. However, the amount of the oil phase as the medium for removal of the carboxylates may be increased by recycling the oil phase into the catholyte following removal by extraction of the carboxylates from the oil phase. The extraction may be effected by procedures such as contacting a portion of or the entire oil phase 18 containing the carboxylates with an aqueous alkaline solution with a pH of 5 or higher or treating the oil phase 18 with an anion exchange resin. Thus, it is possible to maintain the carboxylates in the catholyte at a still lower concentration.

The following examples illustrate the invention:

EXAMPLE 1 An electrolytic hydrodimerization of acrylonitrile for the formation of adiponitrile was run using a catholyte at pH 8 consisting of 25 percent by volume of an oil layer and 75 percent by volume of an aqueous layer.

The catholyte was fed into a rectification tower at a rate of 500 l./hr. to remove acrylonitrile and propionitrile. To the bottom liquor was then added 6N sulfuric acid to adjust the pH to 3.0-2.5. While adjusting the pH, the bottom liquor was stirred for 5 min. and then allowed to stand at 70 C. for 30 min. There were separated the oil phase and aqueous phase. A

sample was taken at each stage of the step and gaschromatographically analyzed to give the results shown in the table below.

Thus, the concentration of carboxylates in the catholyte could be maintained at 60 ppm on average throughout continuous operation for about 1,000 hours. During the operation the yield of adiponitrile could be maintained at 90.5 percent or higher.

On the other hand, when the removal of carboxylates was not conducted, the carboxylate concentration in the catholyte became over 300 ppm and the yield of adiponitrile was lowered to 86 percent.

EXAMPLE 2 The same procedures as in Example 1 were repeated using a double volume of each of the distillation tower feed, distillate and bottom liquor, the separated oil and aqueous phases and 6N sulfuric acid. A half volume of the resulting separated oil phase 18 was countercurrently extracted with one third volume of aqueous sodium carbonate at pH 10. The electrolysis was operated while recycling the thus obtained oil phase without carboxylates removed to the catholyte tank.

In this experiment the carboxylate concentration in the catholyte could be maintained as low as 30 ppm on average throughout continuous operation for 1,000 hours. During the operation the yield of adiponitrile could be maintained at 92 percent.

EXAMPLE 3 The same procedures as in Example 2 were repeated except that a half volume of the separated oil phase 18 was treated with a hydroxyl anion exchange resin for the removal of carboxylates in place of the countercurrent extraction with aqueous sodium carboxylate. The electrolysis was operated while recycling the oil phase thus obtained to the catholyte tank.

The results were similar to those in Example 2.

We claim:

1. A method of purification of catholyte used in the electrolytic hydrodimerization of acrylonitrile which comprises subjecting a used catholyte containing low-boiling byproducts including propionitrile to distillation to remove said by-products, adjusting the pH of the emulsified bottom liquor consisting of an oil phase and an aqueous phase to 3.0 or below with an aqueous solution consisting of an acid to remove the major portion of carboxylates contained in the aqueous phase into the oil phase mainly consisting of adiponitrile, and recycling the resulting purified aqueous phase into the catholyte.

2. A method according to claim 1 wherein the carboxylates are removed from said oil phase and the resulting purified oil phase is recycled into the catholyte.

3. A method according to claim 2 wherein the carboxylates are removed from the oil phase to which said carboxylates have been distributed by extracting the same with an aqueous solution at pH 5 or higher.

4. A method according to claim 2 wherein the carboxylates are removed from the oil phase to which said carboxylates have been distributed by treating the same with an anion exchange resin.

tained at a pH from 3.0 to 2.0.

6. A method according to anyone of claim 1 wherein sulfuric acid is added to the remaining emulsified liquor to adjust 5. A method according to anyone of claim 1 wherein pH of 5 PH ofthe aqueous phase to or below the aqueous phase in the remaining emulsified liquor is maini i l 

2. A method according to claim 1 wherein the carboxylates are removed from said oil phase and the resulting purified oil phase is recycled into the catholyte.
 3. A method according to claim 2 wherein the carboxylates are removed from the oil phase to which said carboxylates have been distributed by extracting the same with an aqueous solution at pH 5 or higher.
 4. A method according to claim 2 wherein the carboxylates are removed from the oil phase to which said carboxylates have been distributed by treating the same with an anion exchange resin.
 5. A method according to anyone of claim 1 wherein pH of the aqueous phase in the remaining emulsified liquor is maintained at a pH from 3.0 to 2.0.
 6. A method according to anyone of claim 1 wherein sulfuric acid is added to the remaining emulsified liquor to adjust pH of the aqueous phase to 3.0 or below. 